The reversible integration of a plasmid into a phage genome via int-mediated recombination permits rapid and facile analysis of plasmid genes. Seventy-six independent mutants in the β-lactamase (bla) gene have been isolated and genetically mapped. The end points of deletions generated by partial digestion with restriction enzymes have been identified and this, together with the assignment of some nonsense mutations to specific amino acid residues, allows alignment of the genetic map with the DNA sequence.